These are called morphisms in [1]} and [2]}. We write \((A_0,B_0,R_0)\le _B(A_1,B_1,R_1)\)  if there is a Borel Tukey reduction from \((A_0,B_0,R_0)\)  to \((A_1,B_1,R_1)\) . The following lemma is immediate from the definitions.
